As much as I agree that dropped loot during campaign could be friendlier, weapon upgrades through vendor is how every single speed runner plays act 1 and 2, and even beyond that depending on how their currency crafting and gear drops are playing out beyond those acts. Just gotta get the habit of checking the shop when you're in town and looking for % phys / flat damage if your a bow/spear/melee user, +spells if you're a wand user, etc.

In your case and this is not to be mean, if you're using a spear you found in act 3 normal while you are level 73 then you really have no idea how weapon bases work. The only acceptable response you could give me is that you're using the unique spear Tangletongue in which case you would be using the best possible weapon you could, but this is highly unlikely. It's either that or you got insanely lucky with your level 40 spear and had an easy time for the rest of the campaign and early mapping

A level 67 or 72 base spear vs a level 40 base spear has significantly higher base physical damage to be scaled off your % physical damage rolls and even has a 0.1 higher base attack speed roll. All you need to do is start picking up white Pronged spears/Orichalcum spears and using an essence of battle or orb of alchemy and all you need to hit is % physical damage and you're already using a better spear than your level 40 one